1. Field of the Invention
The present invention relates to a method and apparatus for calculating correction values for thermal displacement occurring along an axis of a main spindle in a machine tool.
2. Description of the Prior Art
A machine tool has sources of the heat generation at respective portions, and from these sources of the heat generation, heat is transmitted and causes the respective portions of the machine tool to thermally deform. Particularly, the bearing rotatably holding the main spindle generates heat due to rolling of a rolling element, and the main spindle expands, and this causes machining errors. FIG. 4 shows the condition where the main spindle thermally displaces (expands) with time after the main spindle is rotated. As shown in the figure, after the rotation is started, the main spindle expands at a predetermined time constant, and then turns into a steady state soon thereafter.
Conventionally, a liquid coolant is supplied and circulated in a housing that holds the main spindle to cool the bearing, and in addition, the temperature in the vicinity of the main spindle is measured, the amount of thermal displacement of the main spindle is estimated on the basis of the measured temperature, and a correction value is calculated on the basis of the estimated thermal displacement amount, and tool offset is made on the basis of the calculated correction value, whereby the thermal displacement is canceled.
In a running condition where the main power supply of the machine tool is turned on, the main spindle is not always rotated, but occasionally stopped. For example, in the case of a machining center, when a tool that has reached the wearing limit is replaced with a new one, the rotation of the main spindle must be interrupted in order to set a tool offset value of the replaced tool. Also, in a case where a workpiece after being machined is measured on the machine tool, the rotation of the main spindle must be interrupted. When the rotation is interrupted, the main spindle is cooled, and as shown by the chain double-dashed line in FIG. 4, the amount of thermal displacement decreases, that is, the main spindle contracts.
However, as mentioned above, in a machine tool constructed so that the bearing is cooled by a liquid coolant, the cooling condition in the vicinity of the main spindle to be measured, that is, the cooling condition in the vicinity of the bearing does not always agree with the cooling condition of the main spindle itself, so that a phenomenon in which the main spindle is cooled later occurs. Therefore, the actual contraction amount of the main spindle is smaller than the contraction amount estimated on the basis of the measured temperature in the vicinity of the main spindle. In other words, the actual contraction of the main spindle is delayed from the estimated contraction on the basis of the measured temperature in the vicinity of the main spindle. This condition is shown in FIG. 5. In FIG. 5, the actual contraction condition of the main spindle is shown by the chain double-dashed line, and the estimated contraction condition on the basis of the measured temperature in the vicinity of the main spindle is shown by the continuous line.
When the main spindle is re-rotated, the temperature of the main spindle rises again due to heat generated from the bearing. This condition is shown in FIG. 6. In FIG. 6, in the same manner as mentioned above, the actual contraction and expansion conditions of the main spindle are shown by the chain e double-dashed line, and the estimated contraction and expansion conditions on the basis of the measured temperature in the vicinity of the main spindle are shown by the continuous line. As shown in FIG. 6, in the condition where the main spindle is re-rotated, the actual contraction amount of the main spindle has a delay amount xcex94Z to the contraction amount estimated on the basis of the measured temperature in the vicinity of the main spindle, and thereafter, with the elapse of time, the main spindle temperature and the temperature in the vicinity of the main spindle reach the same temperature, and the abovementioned delay amount converges at zero.
As mentioned above, when the main spindle is re-rotated, the actual contraction amount of the main spindle is delayed by xcex94Z to the estimated contraction amount on the basis of the measured temperature in the vicinity of the main spindle. Therefore, when the correction value is calculated on the basis of the estimated contraction amount, an error corresponding to the delay amount xcex94Z to the actual contraction amount of the main spindle occurs, so that accurate correction cannot be made.
In a case where a tool offset value is set for a replaced tool or a workpiece after being machined is measured on the machine tool during the interruption of the rotation of the main spindle, if correction is made on the basis of the estimated contraction amount having an error to the actual contraction amount of the main spindle, accurate setting of the tool offset value and accurate measurements of the workpiece cannot be made.
The invention is made in view of the abovementioned circumstances, and the object thereof is to provide a method and apparatus for calculating a correction value for thermal displacement in a machine tool whereby accurate correction values can be calculated when the rotation of the main spindle is interrupted or the rotation of the main spindle is restarted.
In order to solve the abovementioned problems, the present invention relates to a method for calculating a correction value for thermal displacement occurring along an axis of a main spindle in a machine tool, the method comprising the steps of:
measuring a temperature in the vicinity of the main spindle;
estimating an amount of the thermal displacement on the basis of the measured temperature and a predetermined thermal displacement amount calculation expression; and
calculating the correction value on the basis of the estimated thermal displacement amount,
wherein, when the main spindle is continuously rotated without interruption after start-up thereof, the correction value is calculated on the basis of the estimated thermal displacement amount alone; and
wherein, when the rotation of the main spindle is restarted after interruption thereof, an amount of displacement delay occurring due to the interruption is estimated, and the correction value is calculated on the basis of the estimated displacement delay amount and the estimated thermal displacement amount.
In the invention, the amount of thermal displacement occurring along the axis of the main spindle on the basis of the measured temperature in the vicinity of the main spindle is estimated. After the main spindle is started up, when the main spindle is continuously rotated without interruption, a correction value is calculated on the basis of the estimated thermal displacement amount alone, and on the other hand, when the rotation of the main spindle is restarted after interruption thereof, the amount of displacement delay occurring due to the interruption is estimated, and a correction value is calculated on the basis of the estimated displacement delay amount and the estimated thermal displacement amount. Therefore, as mentioned above, when the rotation of the main spindle is restarted after interruption thereof, even in a case where the actual contraction amount (displacement amount) of the main spindle has a delay amount to the contraction amount (displacement amount) estimated on the basis of the measured temperature in the vicinity of the main spindle, a correction value is calculated on the basis of the delay amount as described above, whereby accurate correction can be made.
To calculate a correction value on the basis of the displacement delay amount and the estimated thermal displacement amount can be carried out by the following, that is, an adjusting displacement amount is calculated on the basis of the estimated displacement delay amount, and the correction value is calculated on the basis of a displacement amount obtained by addition or subtraction of the adjusting displacement amount to/from the estimated thermal displacement amount.
On the supposition that the adjusting displacement amount is xcex4c, this amount can be calculated from the following expression.
xcex4c=xcex94Zxc3x97exp(xe2x88x92tc/Tc) 
(In this expression, xcex94Z is the displacement delay amount (xcexcm) at the restart of the rotation of the main spindle, Tc is a time constant, and tc is a time (s) elapsed from the restart of the rotation of the main spindle.)
The displacement amount obtained by addition or subtraction of the adjusting displacement amount calculated from such a calculation expression to/from the estimated thermal displacement amount is approximate to the actual displacement amount of the main spindle. Therefore, by calculating a correction value on the basis of this displacement amount, a more accurate correction can be made.
The abovementioned method for calculating the correction value can be preferably carried out by the following apparatus. That is, the apparatus for calculating a correction value for thermal displacement occurring along an axis of a main spindle in a machine tool, the apparatus comprising:
temperature measuring means for measuring a temperature in the vicinity of the main spindle;
thermal displacement amount estimating means for estimating an amount of the thermal displacement on the basis of the temperature measured by the temperature measuring means and a predetermined thermal displacement amount calculation expression;
correction value calculating means for calculating the correction value on the basis of the thermal displacement amount estimated by the thermal displacement amount estimating means; and
adjusting displacement amount calculating means for estimating an amount of displacement delay occurring when rotation of the main spindle is restarted after interruption thereof, and calculating an adjusting displacement amount on the basis of the estimated displacement delay amount;
wherein, when the main spindle is continuously rotated without interruption after start-up thereof, the correction value calculating means calculates the correction value on the basis of the thermal displacement amount estimated by the thermal displacement amount estimating means alone;
wherein, when the rotation of the main spindle is restarted after the interruption thereof, the correction value calculating means calculates a displacement amount by adding or subtracting the adjusting displacement amount calculated by the adjusting displacement amount calculating means to/from the thermal displacement amount estimated by the thermal displacement amount estimating means, and calculates the correction value on the basis of the displacement amount.
The adjusting displacement amount calculating means can be constructed so as to calculate the adjusting displacement amount xcex4c from the following expression.
xcex4c=xcex94Zxc3x97exp(xe2x88x92tc/Tc) 
(In this expression, xcex94Z is the displacement delay amount (xcexcm) at the restart of the rotation of the main spindle, Tc is a time constant, and tc is a time (s) elapsed from the restart of the rotation of the main spindle.)
In accordance with the method of the invention for calculating a correction value for thermal displacement, when the rotation of the main spindle is interrupted, an amount of displacement occurring along the axis of the main spindle during the interruption is estimated on the basis of an amount of the thermal displacement estimated on the basis of a temperature measured immediately before the interruption and a predetermined delay amount to the estimated thermal displacement amount, and the correction value can be calculated on the basis of the estimated displacement amount.
According to the invention, when the rotation of the main spindle is interrupted, an amount of displacement occurring along the axis of the main spindle during the interruption is estimated on the basis of an amount of the thermal displacement estimated on the basis of a temperature measured immediately before the interruption and a predetermined delay amount to the estimated thermal displacement amount, that is, a displacement amount approximate to the actual displacement amount of the main spindle during the interruption is estimated, and the correction value during the interruption is calculated on the basis of this estimated displacement amount. Therefore, a more accurate correction can be made for thermal displacement of the main spindle during the interruption of the rotation. Thereby, setting of a tool offset value after replacing the tool or workpiece measurements on the machine tool during the interruption can be made with high accuracy.
The method of the invention can be preferably carried out by the following apparatus. That is, the apparatus is constructed so that, when the rotation of the main spindle is interrupted, the correction value calculating means estimates an amount of displacement occurring along the axis of the main spindle during the interruption on the basis of an amount of the thermal displacement estimated on the basis of a temperature measured immediately before the interruption and a predetermined delay amount to the estimated thermal displacement amount, and calculates the correction value on the basis of the estimated displacement amount.
The amount xcex4B of the displacement occurring along the axis of the main spindle during the interruption can be calculated from the following expression.
xcex4B=xcex4A1xc3x97exp(xe2x88x92tB/TB) 
(In the above expression, xcex4A1 is the estimated thermal displacement amount immediately before the interruption of the rotation of the main spindle, TB is a time constant, and tB is a elapsed time from interruption of the rotation of the main spindle.)